The present invention relates to a magnetic memory device and a method for manufacturing the same.
In recent times, a variety of memories have been widely used throughout the world, in particular, dynamic RAMs (DRAMs) have the largest circulation market in association with such memories. A DRAM is a representative memory device in which one MOS transistor and one capacitor create a pair used to store one bit. This DRAM is a volatile memory device, which stores data as an electric charge in a capacitor. This requires a periodic refresh operation so as not to lose stored data.
In contrast to the above DRAM, a NAND/NOR flash memory is used as a non-volatile memory like a hard disc drive (HDD), which does not lose stored data even when power is turned off. Particularly, the NAND flash memory has the highest degree of integration among commercial flash memories. A device using the flash memory can be designed smaller and lighter than ones using a HDD. In addition, the flash memory has very strong resistance to a physical impact, has a very fast access speed, and consumes a small amount of power, so that it is generally used as a storage medium for mobile products. However, the flash memory has slower speeds and a higher operation voltage as compared to the DRAM.
In recent times, many people and companies are conducting intensive research into a method for manufacturing an improved memory having the advantages of both the flash memory and the DRAM, and allowing the improved memory to be commercially available. Representative examples of the improved memory are a phase change RAM (PCRAM), a magnetic RAM (MRAM), a polymer RAM (PoRAM), a resistive RAM (ReRAM), and the like.
Specifically, the MRAM is designed to use a resistance variation caused by a polarity variation of a magnetic body as a data storage method. Some low-capacity MRAM products among MRAMs have already been commercially manufactured and introduced to the market. Furthermore, the MRAM is a memory based on magnetism, so that it is not damaged even by radioactivity from space. As a result, it is considered that the MRAM has been considered the safest and most stable among memories above.
FIG. 1 is a cross-sectional view illustrating a conventional MRAM structure.
A write operation of the MRAM is carried out by the vector sum of two magnetic fields from two currents simultaneously flowing in a bit line (BL) and a digit line (DL). The first magnetic field is generated by the current flowing through bit line (BL) and the second magnetic field is generated by the current flowing through digit line (DL).
In other words, the conventional MRAM illustrated in FIG. 1 must include not only a bit line but also a digit line. As a result, the conventional MRAM cell increases in size, such that the cell efficiency is deteriorated as compared to other memories.
The MRAM has another disadvantage in that a half-selection status may occur in a process for selecting a cell and writing data to that selected cell. This can cause unselected cells sharing the digit line (DL) or the bit line (BL) to be exposed to a magnetic field generated from a corresponding line due to the occurrence of the half-selection status. In this case, a disturbance in which a status of unselected cell is inverted may occur during a write operation.
Accordingly, a smaller-sized MRAM that does not need to use the digit line (DL) has recently been developed, and a spin transfer torque (STT)—magnetic RAM (MRAM) capable of preventing a disturbance caused by the half-selection status in a write operation is being developed. In more detail, if a spin transfer torque (STT) phenomenon occurs in a ferromagnetic body (i.e., if a high-density current signal is incident to a ferromagnetic body) and a magnetization direction of the ferromagnetic body does not coincide with a spin direction of the current signal, a magnetization direction of the ferromagnetic body is aligned with the spin direction of the current signal. This phenomenon is called a STT phenomenon, such that the STT-MRAM is designed using characteristics of the STT phenomenon.